An infrared sensor can detect infrared light radiated from an object such as a human body. With the ability for non-contact detection of the existence or the temperature of an object, it has been expected to find applications in a variety of fields of technology. Particularly, an infrared sensor including a plurality of infrared sensor units arranged in a matrix pattern is capable of obtaining a two-dimensional infrared light image, and has been expected to find applications in an even wider variety of fields of technology. Favorable candidates for such infrared sensors include dielectric bolometers detecting a change in the dielectric constant caused by a temperature change through the application of an electric field, because they do not require cooling or a chopper circuit.
FIG. 12 shows a signal reading circuit of a known dielectric bolometer-type infrared sensor. As shown in FIG. 12, a series capacitor element 201 and an infrared-detecting capacitor element 202 are connected in series with each other via a node 210. The infrared-detecting capacitor element 202 has characteristics such that the capacitance thereof varies depending on the intensity of infrared light incident on the element. The characteristics of the element are set so that the capacitance value of the infrared-detecting capacitor element 202 and that of the series capacitor element 201 are equal to each other when there is no infrared light incident thereon.
An alternating-current power supply 204 and an alternating-current power supply 205 are connected to the series capacitor element 201 and the infrared-detecting capacitor element 202 for driving the capacitor elements 201 and 202, respectively, wherein the alternating-current power supply 204 and the alternating-current power supply 205 have the same amplitude and inverted phases.
The node 210 is connected to an output terminal 206 via a transistor 203, and the potential of the node 210 can be taken out to the output terminal 206 by turning ON the transistor 203 via a signal line SSW.
The potential of the node 210 is determined by the capacitance values of the series capacitor element 201 and the infrared-detecting capacitor element 202 and the voltages (amplitudes) of the alternating-current power supply 204 and the alternating-current power supply 205. Therefore, when infrared light is incident on the infrared-detecting capacitor element 202, whereby the capacitance value of the infrared-detecting capacitor element 202 increases as shown in FIG. 13, there is obtained an output curve as shown by an output curve A in FIG. 13. In FIG. 13, a curve C and a curve D represent output voltages of the alternating-current power supply 204 and the alternating-current power supply 205, respectively.
When there is no infrared light incident on the infrared-detecting capacitor element 202, the capacitance value of the series capacitor element 201 and that of the infrared-detecting capacitor element 202 are equal to each other, whereby the potential of the node 210 is always zero as indicated by B in FIG. 13. In view of the above, infrared light can be detected with high accuracy (see, for example, Patent Document 1). Patent Document 1: Japanese Unexamined Application Publication No. 2002-365130